This invention is related to the reinforcing of vinyl chloride homopolymers ("VC homopolymers" for brevity) with glass fibers in the field of improving the physical strength characteristics of organic synthetic resinous materials. More particularly, this invention is related to thermoplastic glass fiber reinforced ("GFR") poly(vinyl chloride) ("PVC") homopolymer, and chlorinated poly(vinyl chloride) ("CPVC") which are either individually or together referred to herein as "VC homopolymer" and commonly available as rigid PVC and CPVC.
Currently there are available numerous GFR thermoplastic materials including polyolefins, polyacetals, polyamides (nylons), polycarbonates, polystyrenes, styrene-acrylonitrile copolymers, acrylonitrile-butadiene-styrene (ABS) copolymers, and most recently, PVC. These GFR polymers are used in various forms, chief amongst which are generally spherical or ellipsoidal molding pellets having an equivalent diameter in the range from about 3 mm to about 8 mm, used to feed a thermoforming machine. By "thermoforming" I refer to the transformation of thermoplastic resin into a useful shape by means of heat and/or pressure. Illustrative thermoforming processes are molding by injection of hot resin into a mold, extrusion, pultrusion, hot calendering, casting, vacuum forming and the like. Chopped glass fibers less than about 6.4 mm long are fed to the machines in which the vigorous mixing and high shear flow patterns cause further comminution often reducing the length 10-100 times, so that the principal function of the fibers is to stiffen rather than strengthen a composite (see Encyclopedia of Chemical Technology by Kirk & Othmer, pg 973, Vol 13 Third Ed., John Wiley & Sons, 1981). This invention is concerned with strengthening GFR PVC and CPVC composites, not only with respect to their dry strength but also to their wet strength.
Since the largest volume general purpose thermoplastic resin commercially today produced in the world is PVC, and has been for many years during which others of the aforementioned resins have been successfully exploited after they have been reinforced with glass fibers, it would appear anachronistic that the propitious appearance of GFR PVC in the market place should have suffered such an unseemly delay. To a lesser extent this is also true of CPVC which has been used extensively for the extrusion of pipe to carry hot fluids under pressure, especially corrosive liquids; and, for the injection molding of a variety of plumbing fittings, housings for pumps and the like.
Faced with the task of providing a commercially acceptable GFR VC resin one soon reconciles the delay in the debut of the resin in the market place. One finds that PVC lacks thermal stability and, when reinforced with even a relatively low level of glass fiber content, say about 10 percent by weight (% by wt) based on the combined weight of resin and glass, acquires a disconcertingly high viscosity which makes it difficult to fill the corners of even a relatively small mold.
To combat the lack of thermal stability, numerous stabilizers have been incorporated into the VC resins. To reduce the viscosity numerous solutions have been proffered, most logical of which has been the hunting and choosing of various likely copolymers which might compatibly be blended with the VC resin. For example, U.S. Pat. Nos. 2,572,798; 2,773,851; and 3,883,473 disclose blending PVC with a coumarone-indene resin. A conventional process for incorporating glass fibers into various resins is disclosed in U.S. Pat. No. 3,164,563.
Stabilizers in VC molding resins are particularly adapted to combat their dehydrochlorination which is accelerated above the glass transition temperature ("T.sub.g ") of the resin, and is particularly severe above the melting point. This dehydrochlorination of VC resins is well-recognized in the art to give rise to a double bond adjacent to a Cl atom in the VC chain (see the chapter titled "Thermal Degradation and Stabilization" in the text Polymer Stabilization by W. Lincoln Hawkins, pg 126 et seq., Wiley Interscience 1972). This Cl is referred to as an "allylic Cl" and it is generally regarded as being instrumental in the degradation of VC resins. The generation of allylic Cl atoms and concomitant degradation occurs throughout the mass of resin being thermoformed, and not surprisingly, to counteract the degradation, the stabilizer is desirably distributed throughout the mass to negate the propagation of allylic Cl atoms in each chain. Understanding the foregoing mechanism has led to the search for more effective stabilizers which negate propagation more quickly, and for lower processing temperatures.
In view of the known proclivity of GFR VC homopolymer to degrade at elevated temperatures due to the propagation of allylic Cl atoms, it is especially noteworthy that it is these generally undesirable allylic Cl atoms which have now been recognized to be essential to provide a unique reaction between the primary amine group (referred to as a reactive amine moiety) of an aminosilane coupling agent and the VC homopolymer chain. It is this reaction which inculcates unexpected strength in a thermoformed VC homopolymer which is reinforced with glass fibers specifically "sized" to provide the necessary reaction.
Thus, the invention is more generally directed to copolymers of VC with a copolymerizable monomer in which copolymers VC is present in an amount sufficient to generate an allylic Cl atom under thermoforming conditions. The copolymers may be postchlorinated provided there are sufficient runs of 10 or more C atoms in VC chains to generate reactive allylic Cl atoms. Such copolymers of VC, optionally postchlorinated, and VC homopolymer are generically referred to herein as "VC resins".
By a "run of C atoms" we refer to a portion of the polymer chain which is characteristically a PVC chain, that is, having about 57% Cl in the run. As the Cl content of the run increases, as it will when a VC copolymer or homopolymer is chlorinated (`postchlorinated`), the difficulty of generating a reactive allylic Cl atom increases. The presence of runs in a polymer may be identified by nuclear magnetic resonance (NMR) spectra as is taught in U.S. Pat. Nos. 4,350,798 and 4,377,459 to Richard G. Parker the disclosures of which are incorporated by reference thereto as if fully set forth herein.
By the term "sized" or "sizing" I refer to glass fibers, whether in strands, rovings, tow or yarns, which are treated specifically for use in a GFR thermoplastic resin. Unsized glass fibers are also referred to as untreated, `pristine`, or `bare` glass fibers. For use as reinforcing, glass fibers are provided with a "size" which in this invention is the combination of a coupling agent or `finish` and a `film former` without regard to the physical form in which they are combined on the surface of the fibers.
The `finish` or `coupling agent` is typically particularly chosen by the manufacturer of the fibers with particular regard for the specific resin in which the fibers are to be used. Numerous finishes are used, the organosilanes being preferred for general use. Examples of silanes are found in U.S. Pat. Nos. 2,563,288; 2,563,889; 3,318,757; 3,493,461 and many others. Additional finishes are listed in a publication titled "The Influence of Reinforcements on Strength and Performance of Fiber Glass Reinforced Thermoplastics" by J. T. Inglehart et al, given at the 22nd meeting of Reinforced Plastics Division of the Society of Plastic Industry, Inc.
Some GFR composites in which the glass fibers are coated with a coupling agent only, display remarkably improved strength. However, the notion that simply providing a proper coupling agent will maintain the original dry strength of a composite after it is submerged in water for a month, is unfounded and harsh reality belies it.
Of course, persons skilled in the art are well aware that the choice of finish and film former (bonding interlayer) are two of a multiplicity of variables which influence the strength and performance of GFR resins. It is also generally recognized that even with the optimum choice of finish and film former for reinforcing a particular resin, along with the type of glass and the length and diameter of the glass fibers used, one can have too little finish and/or film former; or, too much of either, or too much of both. However, it is immediately apparent tha there are so many choices of combinations of variables to be explored that even extensive not-so-simple trial and error, such as one skilled in the art is enured to, will be a most unlikely method of finding an optimum combination of variables. The task is greatly simplified if one divines the essential physical/chemical mechanism which might lead the way to discovery of the optimum combination of variables.
With this logical approach to providing a better solution to the problem of making a satisfactory if not superior GFR composite with a VC resin, U.S. Pat. No. 3,493,461 teaches that the key variable is choice of the organosilane. A vast array of silanes and hydrolyzates thereof are taught to provide GFR VC composites as long as the silane-treated glass fibers and PVC resins are brought in intimate contact with each other in any convenient manner, and then thermoformed. Buried in the disclosure of a very large number of silanes, and their hydrolyzates deemed to be equally or comparably effective, is the teaching of an aminoalkyltrialkoxysilane (in grouping (c) in col 2) which includes numerous compounds lacking a primary amine reactive moiety. Since the vast majority of the silanes disclosed do not have the essential primary amine reactive moiety which catalyzes generation of the desired allylic Cl atoms, these silanes are incapable of adequately catalyzing the formation of an allylic Cl; and, of course, it will be recognized that the hydrolyzates of these silanes are also incapable of doing so.
Some references, such as U.S. Pat. No. 3,644,271 teach that the key lies in heating and masticating a mixture of a powdered PVC in a specified particle size, in combination with pelletized PVC in the correct size; and, a specified amount of each PVC by weight relative to the weight of glass fibers used; and, that all the other variables are much less important. It even suggests that "The difficulties of obtaining a good bond between the thermoplastic material and the glass fibers has been substantially overcome by the development of suitable treating agents for the glass fibers."
In many prior art references, it is averred that the key lies in providing the correct film former, namely a vinyl chloride-vinyl trialkoxysilane copolymer, and whether the glass fibers are finished or bare does not play an important part in determining the degree of the additional reinforcing effect of the bonding layer (see, e.g. U.S. Pat. No. 3,928,684 col 4, lines 46-53).
No reference has recognized the criticality of generating the allylic Cl atoms in a zone adjacent the surface of the glass fibers where they can react with the primary amine moiety of the coupling agent. Of course, at extrusion temperatures, varying amounts of allylic Cl atoms may be generated solely due to thermal initiation, but these are generated throughout the mass of the hot resin and are immediately negated by the stabilizer present, hence are of little value near the surface of the glass fibers where they are needed. It is the recognition of this phenomenon which provided the impetus to find a coupling agent which had the unique propensity for catalyzing the formation of an allylic Cl in a zone adjacently surrounding the glass fibers.
However, it was found that not only was the generation of allylic Cl atoms by the presence of the aminosilane inadequate, but also the wet strength of a GFR VC resin, either with no film former, or with an arbitrarily chosen film former, was unacceptably so low as to have no commercial significance. By "wet strength" we specifically refer to the strength of a GFR VC resin composite which has been submerged in 50.degree. C. water for at least 900 hr. Thus, it was necessary to bolster both the catalytic action of the aminosilane, and also the wet strength of the GFR VC composite.
It was determined that the selection of the particular components of the sizing was the key to providing the necessary catalytic action and also the wet strength. The identification of these two essential components of the "sizing" for the glass fibers, along with providing the requisite amount of heat to help the generation of the allylic Cl atoms, and controlling the amounts of the finish and film former present on the fibers, pointed the way to the discovery of a composition which is as unobvious as it is uniquely tailored; of a process for forming pellets of the composition adapted for injection molding the composition; and, of shaped articles produced from the composition.